Also, many mutations are completely benign and don't influence anything right now. They confer genetic diversity to a population. Later, if a selective pressure appears via a rapid change in the environment, such as the introduction of a disease, a new predator, or climate change, what was a benign mutation might suddenly confer a survival advantage, and will begin being selected for.

This is why genetic diversity is important. Populations aren't just more copies of a single genome. They are in fact reservoirs of genetic diversity which makes ecosystems more resilient to change. This partly explains why there are so many species of insects. As the recyclers at the bottom of the foodchain, they are the boots on the ground that do the heavy lifting and are subjected to the first shocks. They evolve into many species not only to specialize, but to guard against rapid changes at the bottom of a food web.